ITS service definition
Ramp Metering is a tool used to manage traffic at motorway junctions on a temporary basis usually during times of peak flow. Ramp metering is implemented via the installation of traffic signals at the on-ramps which regulate the flow of traffic joining the motorway or ‘mainline’ during peak or congested periods. It does this by controlling the discharge of vehicles from the onramp, holding vehicles back and breaking up platoons of vehicles, thus reducing the interference of merging vehicles and helping maintain the flow of traffic on the main carriageway. The traffic signals are generally operated in dependence of the currently prevailing traffic conditions on both the main carriageway and the on-ramp.
ITS service objective
The objective of ramp metering is to prevent or delay the onset of flow breakdown on the mainline, maximising throughput, without disrupting the surrounded urban road network. This is achieved by:
Ramp metering is not used directly to deter drivers making short trips but can have the added benefit that it will discourage drivers who do make short trips from using the motorway network.
ITS service radar
During peak or congested periods on the motorway, the addition of traffic from on-ramps causes vehicles to break or change lanes giving rise to higher occupancy and lower headways. Shorter headways cause drivers to reduce their speeds, resulting in a sustained loss of throughput.
This speed reduction often causes following vehicles to brake, resulting in a propagation wave of slowing vehicles that travels back along the line of traffic on the main carriageway upstream of the on-ramp. This speed adjustment can occur over a distance of up to 2 km prior to the on-ramp. During this time more vehicles will be attempting to join the main carriageway, and if vehicles continue to join, the speed on the main carriageway will fall to a point where flow breakdown occurs. Additionally, during peak periods when congestion is increased there may also be a higher risk of accidents.
Ramp metering (RM) is implemented via the installation of traffic signals on the on-ramps which regulate the flow of traffic joining the motorway during peak or congested periods. It does this by controlling the discharge of vehicles from the on-ramp, holding vehicles back and breaking up onramp platoons, thus reducing the interference of merging vehicles and helping maintain the flow of traffic on the main carriageway.
The traffic signals are generally operated in dependence of the currently prevailing traffic conditions on both the main carriageway and the on-ramps.
From the operational perspective, the vision for ramp metering is effective control of onramp vehicles during congested periods, resulting in fewer accidents and maximised mainline throughput. If properly configured and controlled, the on-ramp control has minimal (and controlled) impact on the adjacent road network.
For drivers, encountering ramp metering in an unfamiliar area (i.e. another country) there would not be cause for anxiety as it has a similar look and feel; the driver knows what to expect and how to proceed. Drivers would accept that the small delay experienced on the on-ramp will mean safer, less congested conditions on the motorway.
There is sometimes a need for driver education where the use of ramp metering is new to a region, but essentially the ramp metering relies on the driver obeying mandatory traffic signals.
The purpose of ramp metering is to prevent or delay the onset of flow breakdown on the main carriageway, maximising throughput, without disrupting the urban road network.
This is achieved by:
Pre-signs and distinct traffic signal heads are used to indicate to drivers there is ramp metering in operation. Nevertheless, the systems rely on the driver obeying mandatory traffic signals as would be the case at any signalised junction.
Ramp metering is a traffic management measure designed to reduce the disruption from platoons of vehicles entering the main carriageway at on-ramps. The measure is characterised by:
Relevant complementary information, not included within this ITS service and covered by other ITS services, is:
Ramp metering is implemented via the installation of traffic signals on the on-ramps which regulate the flow of traffic joining the motorway during peak or congested periods. It does this by controlling the discharge of vehicles from the on-ramp, holding vehicles back and breaking up onramp platoons, thus reducing the interference of merging vehicles and helping maintain the flow of traffic on the main carriageway. The traffic signals are generally operated in dependence of the currently prevailing traffic conditions on both the main carriageway and the on-ramps.
The architecture for ramp metering service is made up of the following three functions:
Figure 61 shows a simple situation together with three data communications interfaces which includes the potential for C-ITS service connectivity in the future. Importantly it illustrates the need to interface with a National Access Point in order to meet the obligations of the ITS Directive.
Cooperation with the adjacent network operators and their traffic control systems also need to be considered where appropriate. This can be achieved using a DATEX II reference profile.
Common Look & feel requirements:
It is up to the deploying road operator to ensure that real signs are well and widely understood by the road users
Common look & feel advice:
Area control: control (sub) centre is hierarchically structured and consists of optional control centre, control sub-centre and local control station.
The devices and methodologies for traffic data collection are not covered by this service description. They depend amongst others on the particular used data collection system and are left to the operator to select; provided a reasonable level of accuracy and reliability is guaranteed.
Usually there are vehicle detectors on the on-ramp and main carriageway to measures traffic conditions:
Local Controller / Outstation
The outstation provides control functionality and calculates release rates and the resulting signal timings based on traffic flow. The outstation can be equipped with a remote communication so that administrative functions can be carried out remotely.
Ramp metering controllers typically operate in the following states:
The above states are described in many documents, EURAMP Deliverable 7.5 for example. The controller activates the ramp metering lights in accordance with the algorithms and contains an interface to traffic detectors (on the main carriageway and on-ramps).
The specific combination of switching criteria and threshold values depends on the traffic conditions and control installation, i.e. the system may only be active as long as it is required by the traffic situation; when the traffic situation eases, the system goes into standby mode.
Ramp Metering Algorithms
Appropriate RM algorithms are used to monitor the traffic conditions and regulate traffic flow on the on-ramp onto the main carriageway. It is recommended that all algorithms are configurable.
Fallback / Failsafe
If signals or local controller fails, it is recommended that the central coordinated strategy be able to continue its operation and coordination of available ramps taking the missing ramp into account. If there are communication failures between the control system and the local controller should automatically switch to fail-safe mode.
Ramp metering systems require power supplies and telecommunication systems such as fibre optic cable or mobile/land-based telephone connections to provide links to the traffic operations centres and the signal controllers; remote communications are becoming more commonly used (sufficient bandwidth is required).
Central Control System
Operational Graphical User Interface (GUI): this is recommended to allow easy inspection, maintenance and repair of local signal controllers. A traffic operations centre GUI should allow for easy access to parameters, variables and display during operation. At times, analysis of historic variables may be required (i.e. in cases of errors or reconstruction of previous scenarios) and so archiving facilities are advantageous.
The necessary computing devices may be centralised or decentralised depending on the adopted architecture. It can be of benefit if devices selected are easily scalable and have sufficient computing power to allow for future additions, updates and future strategy changes.
Compliance with EU Delegated Regulations
It is unlikely that the requirements of the EU Delegated regulations for road safety related traffic information and real-time traffic information services will impact greatly on this service. However, if data collected and processed results in safety information, or traffic management information being created which would be of use to the end user, the National Access Point must be used to publish the information.
Information provision standards:
Table 30 gives the Level of Service recommendations for a Ramp Metering service. The background of this concept is descripted in chapter 2.6..
Table 30: Level of Service recommendations for Ramp Metering
The levels show technological advancement in the ITS solutions that can be implemented where appropriate; and can be cross-referenced to the Operating Environments.
The Levels of Service described here are not intended to indicate that by deploying Level C at all sites there will be improved results over Level B deployments, i.e. Level B is not “better” that Level A. The Levels only indicate advanced ITS technologies or techniques.
The Level of Service selected is closely related to the operating environment (traffic characteristics, level of incidents, road use etc.). As stated earlier, ramp metering is highly site specific in nature, i.e. Level A deployments may achieve the desire results in certain circumstances; but in other more complex situations another Level of Service may be more appropriate.
Using the table above, implementers can select the level of service of each element that is most appropriate, i.e. Level A Pre-signing with a Level 3 Metering Strategy.
The sophistication and size of a ramp metering system should reflect the amount of desired improvement and existing conditions. Ramp metering strategies can be based on fixed metering rates (historical), real-time data, or predicted traffic demand. Strategies can be implemented to optimise conditions locally or systemwide. Each control mode has an associated hardware configuration. If ramp control is linked at several junctions, there is greater overall equity. Distinguished by their responsiveness to prevailing traffic conditions, metering systems fall into three categories:
Level of service requirement:
Table 31: Level of Service to Operating Environment mapping table (see also chapter 2.5.3 and ANNEX C)